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Hi Jack, it's perfectly fine to test interaction using a T, it's just that if you have more than 2 levels to a factor, the overall effect is an F. I think the usual way of doing analysis of variance is to first look for an overall effects, and if this one is significant, use post-hoc T tests. (see an analysis of variance table ...this is especially true if there are a lot of levels in a factor eg > 3) This raises the problem of doing a lot of t tests without any correction for that, so it's worth doing the F first ... the very best JB > Dear Jean Baptiste and SPMers > > Do you mean that you can not test for an interaction using T-contrast ? > The only trouble could be in the interpretation of the result or is there some deeper reasons ? > Sincerely > > > > Jack > ________________________________________________________________ > | Jack Foucher Universite Louis Pasteur | > | Institut de Physique Biologique UPRES-A 7004 du CNRS | > | 4 rue Kirschleger Tel: 33 (0)3 88 77 89 90 | > | 67085 STRASBOURG Fax: 33 (0)3 88 37 14 97 | > | France | > | Faster E-mail: [log in to unmask] | > | Other [log in to unmask] | > |_______________________________________________________________ | > > > > -----Message d'origine----- > De: Jean-Baptiste Poline [SMTP:[log in to unmask]] > Date: mardi 7 decembre 1999 18:06 > A: [log in to unmask] > Objet: Re: advice on contrasts > > > Just a small add on to Jesper email : the "global" mood > by verbal fluency interaction contrast would be > an F-contrast of the form > > -1 1 1 -1 0 0 > 0 0 -1 1 1 -1 > > and the "global" effect of mood > > 1 1 -1 -1 0 0 > 0 0 1 1 -1 -1 > > In general, an interaction of effect1 X effect2 where either > effect1 or effect2 has more than 2 level will express itself > as an F-test, as in classical analysis of variance. > best > jb > > > ------------- Begin Forwarded Message ------------- > > Date: Tue, 07 Dec 1999 16:33:43 +0000 > Subject: Re: advice on contrasts > From: Jesper Andersson <[log in to unmask]> > To: "McBride, Alan" <[log in to unmask]>, "[log in to unmask]" > <[log in to unmask]> > X-Unsub: To leave, send text 'leave spm' to [log in to unmask] > X-List-Unsubscribe: <mailto:[log in to unmask]> > > Dear Alan and David, > > >Dear Spm experts, > > > >We have performed an fMRI expt to examine the effects of experimental mood > >induction on verbal fluency activations on bipolar patients and normal > >controls. Three blocks of scans are obtained, one in each mood state ( > >happy, sad, neutral)while subjects perform a verbal fluency task constisting > >of epochs of generation repetition and rest. > > > >We have specified two conditions: repetition and generation, and examined > >contrasts -1 1 in all three mood conditions, to examine main effect of > >verbal fluency.( ie. -1 1 -1 1 -1 1) > > > > Looks fine. > > > To examine the modulation of fluency activations by mood, we have compared > >contrast -1 1 for scans obtained in the individual mood states (eg -1 1 0 0 > >0 0, if the first series of scans is obtained in Happy mood, and compared > >the activations with those associated with contrast (0 0 -1 1 0 0) where the > >second set of scans were obtained following sad mood induction. > > > > What you describe here seem to be some form of anecdotal description of the > task_by_mood interaction. While this mode of analysis may be helpful in > understanding the different activation patterns, you may want to use the > proper interaction contrast for the inferential assesment. This, as you > probably know, is affected by e.g. the contrast (-1 1 1 -1 0 0) which will > show areas exhibiting a larger activation due to generation in happy mood > than in sad mood. > > > To attempt to identify the main effect of mood, we have compared all scans > >obtained in one mood state vs other mood states, eg ( 1 1 -1 -1 0 0). This > >contrast does not identify any regional differences- we have used scaling to > >remove global effects, and wonder whether this may have anything to do with > >the problem > > I dont think global scaling effects this. I think your problem may be that > the main effect of mood has a very low frequency content (i.e. it changes > very slowly with time). Either this has prevented you from using a high > pass filter (HPF) in which case your sensitivity may be a bit poor. > Alternatively you may have (erroneously) used an HPF wich may then have > removed the main effects of mood. > > I the above I have assumed that you have scanned all moods in the same > "session" such that mood effects and session effects are orthogonal. > > > > >Could you advise us on how best to examine the main effect of mood in this > >experiment. > > > > I think I would need some additional information about your design (e.g. > length of epochs of both verbal task and mood) and analysis (e.g. HPF cut > off frequency) to be able to give any advice. > > >We would also be interested in performing a further analysis, constraining > >the comparison of fluency activations across all mood states to areas > >activated by verbal fluency following neutral mood induction, could you also > >help with advice about how best to do this. > > > > I suspect that what you want to do here is to mask the ineraction contrast > (e.g. [-1 1 1 -1 0 0]) with the main effect contrast (i.e. [-1 1 -1 1 0 > 0]). The resulting (uncorrected) p-level is simply the product of the > p-level threshold of the mask, and the display p-level of the resulting > masked (interaction) contrast. You are likely to be most sensitive if you > use a high threshold (small p-value) for the mask and a more liberal > criterion for the interaction contrast itself. > > >With thanks, David Barbenel, Alan McBride > > > > > > Good luck Jesper > > Jesper Andersson > Wellcome Dept. of Cognitive Neurology > 12 Queen Square > London WC1N 3BG > phone: 44 171 833 7484 > fax: 44 171 813 1420 > > ------------- End Forwarded Message ------------- > > %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%